Wavelength dispersion compensator and optical transmission line

ABSTRACT

In the WDM optical transmission, optical signals propagated through an optical transmission line are supplied to a circulator of a wavelength dispersion compensator. The optical signals supplied to an input terminal of the circulator are transmitted to an input and output terminal of the circulator, and inputted to fiber gratings of the reflection type. The optical signals having a specified wavelength is reflected by one of the fiber gratings, again inputted to the input and output terminal of the circulator, and transmitted to the output terminal of the same. Since the incident optical signals are reflected by the fiber gratings situated at different positions, there arises the difference in the time spent in traveling to and from the fiber grating between the optical signals. Accordingly, the wavelength dispersions of the optical signals can be compensated by suitably selecting the positions of the fiber gratings.

FIELD OF THE INVENTION

[0001] The invention relates to a wavelength dispersion compensator andan optical transmission line, and especially to a wavelength dispersioncompensator and an optical transmission line using the aforementionedwavelength dispersion compensator.

BACKGROUND OF THE INVENTION

[0002] In the wavelength division multiplexed (WDM, hereinafter) opticalcommunication, optical signals propagated through an opticaltransmission line (optical fibers) respectively undergo wavelengthdispersions. The wavelength dispersion is a kind of dispersion, andmeans that a shape of an optical pulse propagated through the opticalfiber is deformed or broadened in various ways depending on a slightdifference in the wavelength of the optical pulse, and group velocitiesof the respective optical signals take different values, chiefly becausethe refractive index of the optical fiber varies as a function of thewavelength of a light.

[0003] Accordingly, the wavelength dispersion is a primary factor of alimitation of the transmission distance of the optical communicationsystem, or of a deterioration of the quality of the optical transmissioncaused by the distortion of the optical pulse. Especially, in the longdistance transmission system using Erbium-doped fiber amplifiers, sincethe optical signal is propagated through the optical fiber without beingconverted into an electrical signal from the sending end to thereceiving end, the wavelength dispersion caused in the total length ofthe transmission line is accumulated on the optical signal.

[0004] Moreover, in a case of the long distance transmission system suchas the submarine optical transmission system or of a high bit ratetransmission system, the effect of the accumulation of the wavelengthdispersion becomes a serious problem. Accordingly, the effect of thewavelength dispersion must be compensated by taking some measures, andexplaining concretely, the wavelength dispersion has been thus farcompensated by inserting the dispersion compensation fiber into theoptical transmission line, where the wavelength dispersion of thedispersion compensation fiber should have the same absolute value as anda sign opposite to that of the optical transmission line.

[0005] The structure of the optical transmission line using thedispersion compensation fiber is shown in FIG. 1. In general, thewavelength dispersion of the dispersion compensation fiber 110 and thatof the optical fiber for the signal transmission 120 change inaccordance with the wavelength of the optical signal. Accordingly, ifthe wavelength dispersion of the dispersion compensation fiber 11 hasthe same absolute value as and a sign opposite to that of the opticalfiber for the signal transmission 120 independently of the wavelength,the wavelength dispersion of the optical signal must be compensatedperfectly at any wavelength.

[0006] However, although it is possible to theoretically cancel thewavelength dispersion of the optical fiber for the signal transmission120 by the wavelength dispersion compensation fiber 110 having theaforementioned characteristic, it has been impossible to actuallyprovide the wavelength dispersion compensation fiber 110 which cancelsthe wavelength dispersions of the optical fiber for the signaltransmission throughout all the channels of the WDM optical signals.

[0007] Herein, FIG. 2 shows a dispersion map of the optical signals incase that the dispersion compensation fibers having the positivewavelength dispersion are inserted into the optical transmission linecomposed of optical fibers for the signal transmission having thenegative wavelength dispersion. As shown in FIG. 2, the wavelengthdispersions of the optical signal λ₁ to λ₅ are compensated by thedispersion compensation fibers, and approach zero at a certain intervalof the transmission distance.

[0008] As seen from FIG. 2, although only the wavelength dispersion ofthe optical signal λ₃ return to zero whenever it is compensated by thedispersion compensation fibers, those of the other optical signals λ₁,λ₂, λ₄, λ₅, do not return to zero even when they are compensated by thesame, because the wavelength dispersions of the optical signals vary asfunctions of the wavelengths of the optical signals, and thereby thewavelength dispersions are accumulated on the optical signals dependingon the transmission distance.

[0009] That is to say, although the conventional dispersion compensationfiber can compensate the accumulated wavelength dispersion satisfactoryonly when it is limited within a certain value, it cannot return theaccumulated wavelength dispersion exceeding the certain limit to zero.Namely, the range of the accumulated wavelength dispersion which can besatisfactorily compensated by the conventional dispersion compensationfiber is limited.

[0010] Moreover, following disadvantages are pointed out on theconventional dispersion compensation fiber. That is to say, an insertionloss of the conventional dispersion compensation fiber becomes large asthe wavelength dispersion to be compensated becomes large. Thedispersion compensation fiber necessitates a length which isproportional to the length of the optical transmission line, and theweight thereof becomes large. Accordingly, it becomes difficult to makethe dispersion compensator using the conventional wavelength dispersioncompensation fiber and devices concerned therewith compact andlightweight, and to reduce consumed electric power and the cost pricethereof.

[0011] Although it can be considered that the accumulated wavelengthdispersions are compensated in the lump at the transmitting andreceiving terminals (not shown), since the accumulated wavelengthdispersion exceeding a certain value cannot be compensated similarly tothe conventional dispersion compensation fiber, it is impossible tocompensate the accumulated wavelength dispersions throughout all thecannels of the WDM optical signals.

SUMMARY OF THE INVENTION

[0012] Accordingly, it is an object of the invention to provide awavelength dispersion compensator and an optical transmission line inwhich a wavelength dispersion can be compensated throughout a wide rangeby a wavelength dispersion compensator having a simple structure, awavelength dispersion compensator can be made compact, consumed electricpower can be reduced, and a quality of a transmission of an opticaltransmission line can be heightened.

[0013] According to the first feature of the invention, a wavelengthdispersion compensator for compensating wavelength dispersions of WDMoptical signals which have been propagated through an input opticaltransmission line, comprises:

[0014] plural fiber gratings of a refection type which reflect the WDMoptical signals at different positions depending on wavelengths of thereflected optical signals.

[0015] The wavelength dispersion compensator mentioned in the abovecorresponds to claim 1.

[0016] If the wavelength dispersion compensator having theaforementioned structure is adopted, since the incident optical signalsare reflected by the fiber gratings of the reflection type at differentpositions depending on the wavelengths of the reflected optical signals,there arises a difference in the time spent in travelling to and fromthe fiber grating of the reflection type between the reflected opticalsignals, and thereby the wavelength dispersions of the optical signalscan be compensated. Accordingly, since the plural optical signals, thewavelength dispersions of which are satisfactorily compensated, aretransmitted on the optical transmission line, the quality of the signaltransmission of the optical transmission line can be heightened.Moreover, since the weight of the fiber gratings of the reflection typeis lighter than that of the conventional wavelength dispersioncompensation fiber, and the length of the former is shorter that of thelatter, the wavelength dispersion compensator according to the inventioncan be made compact and lightweight, and the cost price thereof can becut down.

[0017] In the wavelength dispersion compensator according to claim 2,the wavelength dispersion compensator further comprises a circulator, aninput terminal of which is connected with the input optical transmissionline, an input and output terminal of which is communicated with theplural fiber gratings of a reflection type, and an output terminal ofwhich is connected with an output optical transmission line or an outputoptical waveguide.

[0018] If the wavelength dispersion compensator having theaforementioned structure is adopted, the wavelength dispersioncompensator can be fabricated simply by combining the circulator withthe plural fiber gratings of the reflection type, and thereby thewavelength dispersions of the plural optical signals propagatedthroughout the input optical transmission line can be compensated.

[0019] In the wavelength dispersion compensator according to claim 3,each of the plural fiber gratings of the reflection type is formed of achirped fiber grating of the reflection type.

[0020] If the wavelength dispersion compensator having theaforementioned structure is adopted, since the chirped fiber grating ofthe reflection type which can be used at any wavelength or cancompensate any amount of the wavelength dispersion can be fabricated,and the chirped fiber grating of the reflection type has such a featurethat the wavelength of the reflected optical signal varies continuouslyas a function of a longitudinal distance, the wavelength dispersioncompensator according to the invention can compensated the wavelengthdispersions throughout a wide range of the wavelength.

[0021] In the wavelength dispersion compensator according to claim 4,the wavelength dispersion compensator further comprises a gainequalizer, wherein:

[0022] an input terminal of the gain equalizer is connected with theoutput terminal of the circulator, and an output terminal of the gainequalizer is connected with the output optical transmission line or theoutput optical waveguide.

[0023] If the wavelength dispersion compensator having theaforementioned structure is adopted, unevenness of the levels of theoptical signals reflected from the plural fiber gratings of thereflection type can be equalized. Accordingly, the accumulatedwavelength dispersion of the optical signals can be compensated and thesignal levels of the same can be equalized by the wavelength dispersioncompensator according to the invention.

[0024] In the wavelength dispersion compensator according to claim 5,the wavelength dispersion compensator further comprises a seriesconnection of an optical repeater and a gain equalizer, wherein:

[0025] an input terminal of the optical repeater is connected with theoutput terminal of the circulator, an input terminal of the gainequalizer is connected with an output terminal of the optical repeater,and an output terminal of the gain equalizer is connected with theoutput optical transmission line or the output optical waveguide.

[0026] If the wavelength dispersion compensator having theaforementioned structure is adopted, some of the optical signalsreflected from the fiber gratings of the reflection type, powers ofwhich are lower than their rated powers assigned by the level diagram,can be amplified by the optical repeater to achieve their regularvalues.

[0027] Moreover, since the gain equalizer is connected in series withthe optical repeater, the accumulated wavelength dispersions of therespective optical signals are compensated, the powers of the same areamplified, and the signal levels of the same are equalized. Accordingly,the quality of the signal transmission of the optical transmission linecan be heightened.

[0028] According to the second feature of the invention, the opticaltransmission line according to claim 6 comprises:

[0029] plural wavelength dispersion compensators, each of whichcompensates a wavelength dispersion of a single optical signal,

[0030] a demultiplexing arrayed waveguide grating (AWG, hereinafter)which demultiplexes WDM optical signals into plural optical signals, andsupplies them to the plural wavelength dispersion compensatorsrespectively, and

[0031] a multiplexing AWG which multiplexes the plural optical signalsrespectively outputted from the plural wavelength dispersioncompensators,

[0032] wherein each of the plural wavelength dispersion compensatorscomprises:

[0033] a fiber grating of the reflection type for reflecting one of theplural optical signals, and

[0034] a circulator, an input terminal of which is communicated with thedemultiplexing AWG, an input and output terminal of which iscommunicated with the fiber grating of the reflection-type, and anoutput terminal of which is communicated with the multiplexing AWG,

[0035] wherein optical distances between the fiber gratings of thereflection type and the corresponding circulators take different valuesdepending on wavelengths of the reflected optical signals in the pluralwavelength dispersion compensators.

[0036] If the optical transmission line having the aforementionedstructure is adopted, since the plural wavelength dispersioncompensators are respectively provided for the plural optical signalsobtained by demultiplexing the WDM optical signals, the wavelengthdispersions can be compensated throughout a wide range of thewavelength.

[0037] In the optical transmission according to claim 7, the fibergrating of the reflection type of each of the plural wavelengthdispersion compensators is formed of a chirped fiber grating of thereflection type.

[0038] The advantage of this structure is similar that of the wavelengthdispersion compensator according to claim 3.

[0039] In the optical transmission line according to claim 8, theoptical transmission line further comprises gain equalizers, wherein:

[0040] input terminals of the gain equalizers are respectively connectedwith the output terminals of the circulators, and output terminals ofthe gain equalizers are respectively communicated with the multiplexingAWG.

[0041] The advantage of this structure is similar to that of thewavelength dispersion compensator according to claim 4.

[0042] In the optical transmission line according to claim 9, theoptical transmission line further comprises optical repeaters connectedwith gain equalizers in series, wherein:

[0043] input terminals of the optical repeaters are respectivelyconnected with the output terminals of the circulators, output terminalsof the optical repeaters are respectively connected with input terminalsof the gain equalizers, and output terminals of the gain equalizers arerespectively communicated with the multiplexing AWG.

[0044] The advantage of this structure is similar to that of thewavelength dispersion compensator according to claim 5.

[0045] In the optical transmission line according to claim 10, furthercomprises Co doped fibers inserted between the wavelength dispersioncompensators and the multiplexing AWG.

[0046] If the optical transmission line having the aforementionedstructure is adopted, since some of the optical signals outputted fromthe plural wavelength dispersion compensators, each having a high signallevel, are propagated through the Co fibers and undergo insertionlosses, the levels of the respective optical signals are equalized.

[0047] In the optical transmission line according to claim 11, furthercomprises optical amplifiers respectively inserted between thewavelength dispersion compensators and the multiplexing AWG.

[0048] If the optical transmission line having the aforementionedstructure is adopted, since the optical signals, the accumulatedwavelength dispersions of which are respectively compensated by theplural wavelength dispersion compensators, are so amplified by theoptical amplifiers that the peak levels of the optical signals areequalized, and the outputs of the optical amplifiers are supplied to themultiplexing AWG, the deflections of the respective signal levels can becorrected easily.

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The invention will be explained in more detail in conjunctionwith appended drawings, wherein:

[0050]FIG. 1 shows a structure of a conventional optical transmissionline schematically,

[0051]FIG. 2 is a diagram for showing accumulated wavelength dispersionsof optical signals in a conventional optical transmission line asfunctions of a transmission distance,

[0052]FIG. 3 shows a structure of a wavelength dispersion compensatoraccording to the invention schematically,

[0053]FIG. 4 shows a structure of another wavelength dispersioncompensator according to the invention schematically,

[0054]FIG. 5 shows a structure of the other wavelength dispersioncompensator according to the invention schematically,

[0055]FIG. 6 is a diagram for showing wavelength dispersions of opticalsignals in an optical transmission line using a wavelength dispersioncompensator according to the invention as functions of a transmissiondistance,

[0056]FIG. 7 shows a structure of an optical transmission line accordingto the invention schematically,

[0057]FIG. 8 shows a structure of another optical transmission lineaccording to the invention schematically,

[0058]FIG. 9 shows a structure of the other optical transmission lineaccording to the invention schematically.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0059] Hereafter, preferred embodiments of the invention will beexplained referring to the appended drawings.

Structure of Wavelength Dispersion Compensator

[0060] First, a wavelength dispersion compensator according to thepreferred embodiment of the invention will be explained referring toFIG. 3. This drawing shows an internal structure of the wavelengthdispersion compensator according to the invention.

[0061] As shown in FIG. 3, the wavelength dispersion compensator 10according to the invention is composed of plural fiber gratings of thereflection type 11 and a circulator 12. Herein, the fiber grating of thereflection type 11 is formed of an optical fiber, the refractive indexof which varies periodically as a function of the longitudinal distance,and the period of variation of the refractive index is less than 1 μm.According to the aforementioned structure, the fiber grating of thereflection type 11 functions as a band rejection filter having anextremely narrow band width. That is to say, the fiber grating of thereflection type 11 shown in FIG. 3 reflects an incident optical signalhaving a specified wavelength in the opposite direction, and the otheroptical signals pass therethrough without hindrance.

[0062] The wavelength of the optical signal reflected by the fibergrating of the reflection type 11 is given by the following equation.

λ_(B)=2n _(eff) A(μm),

[0063] wherein

[0064] λ_(B): the wavelength of the reflected optical signal,

[0065] n_(eff): the effective refractive index of the optical fiberforming the fiber grating, and

[0066] A: a coefficient determined by a variation of the refractiveindex of the optical fiber forming the fiber grating (μm).

[0067] In the above equation, when the fiber grating of the reflectiontype 11 is used in the 1.55 μm band, A is about 0.5 μm. Accordingly, thefiber grating of the reflection type 11 serves as a band rejectionfilter having a center wavelength of λ_(B) and a band width of about 1nm or below.

[0068] The number of the fiber gratings of the reflection type 11 is thesame as that of the optical signals. The fiber gratings of thereflection type 11 are connected in cascade by an optical fiber 16 or anoptical wave guide, and an input end (the left end) of the optical fiber16 is connected with a port 12-2 (an input and output terminal) of thecirculator 12. As seen from FIG. 3, since the optical signals arerespectively reflected by the fiber gratings of the reflection type 11which are situated at different positions depending on the wavelengthsof the optical signals to be reflected, in other words, since the lengthof time in which the optical signal travels from the fiber grating ofthe reflection type 11 to the circulator 12 varies depending on thewavelength of the reflected optical signal, the wavelength dispersionsaccumulated on the plural optical signals can be compensatedsatisfactorily by suitably determining the positions of the fibergratings of the reflection type 11.

[0069] Furthermore, in case that the fiber gratings of the reflectiontype 11 is used in the wavelength dispersion compensator 10, it isnecessary to design the amount of variation of the refractive index, theperiod of the same, and the total length of this device should bedetermined so that the fiber grating of the reflection type 11 meets thecharacteristics required thereto.

[0070] Moreover, a chirped fiber grating (not shown) may be applied tothe fiber grating of the reflection type 11. The chirped fiber gratingis a kind of the fiber grating of the reflection type 11, in which theperiod of variation of the refractive index of the fiber varies as afunction of the longitudinal distance. According to the aforementionedstructure, since the center wavelength of the reflection band variescontinuously depending on the period of variation of the refractiveindex, the band width of the chirped fiber grating becomes wider thanthat of the ordinary fiber grating, and the band width of several toseveral tens nm can be obtained.

[0071] In case that the chirped fiber grating is applied to thewavelength dispersion compensator, the number of the chirped fibergrating is the same as that of the optical signals, and the chirpedfiber gratings are connected in cascade by an optical fiber or anoptical waveguide similarly to the ordinary fiber gratings of thereflection type 11. Each optical signal is reflected by thecorresponding chirped fiber grating of the reflection type situated atan appropriate position, and its wavelength dispersion is compensated.

[0072] The function of the circulator 12 is used in the optical circuitshown in FIG. 3 is similar to that used in a microwave circuit, and theport 12-1 (an input terminal), the port 12-2 (an input and outputterminal), and the port 12-3 (an output terminal) are arrangedsymmetrically therearound. In case that the optical signal istransmitted between the adjacent terminals, if the direction of thepropagation of the optical signal coincides with an arrow on thecirculator 12 (from the port 12-1 to the port 12-2, from the port 12-2to the port 12-3, from the port 12-3 to the port 12-1), the insertionloss of the optical signal is quite low. On the other hand, if thedirection of the propagation of the optical signal is opposite to thearrow, the insertion loss of the optical signal is extremely high.Although the circulator 12 having the three terminals is applied to thewavelength dispersion compensator shown in FIG. 3, the circulator havingthe four or more terminals may be applied thereto. In FIG. 3, theoptical signal reflected from the waveguide gratings of the reflectiontype 11 again in inputted to the port 12-2 (the input and outputterminal) of the circulator 12, and transmitted to the output opticaltransmission line via the port 12-3 (the output terminal) of the circuit12.

[0073] In the wavelength dispersion compensator 10 shown in FIG. 4, again equalizer 14 may be connected with the port 12-3 (the outputterminal) of the circulator 12 via an optical fiber or an opticalwaveguide. The gain equalizer 14 equalizes unevenness of the opticalsignal levels, which take different values depending on the wavelengthsof the optical signals. Since the respective optical signals undergodifferent transmission losses which take different values depending onthe wavelengths of the optical signals while they propagate through theoptical fiber for transmission 30, the levels of the optical signals arelowered ununiformaly.

[0074] Accordingly, if the gain equalizer 14 is added to the wavelengthdispersion compensator 10, unevenness of the levels of the opticalsignals which are propagated through the optical fiber for the signaltransmission 30 and reflected by the fiber gratings of the reflectiontype 11 are equalized by the gain equalizer 14, and the optical signalsoutputted form the gain equalizer 14 can be supplied to the opticaltransmission line 1.

[0075] As mentioned in the above, the wavelength dispersion compensator10 shown in FIG. 4 completes equalization of the levels of the opticalsignals as well as the dispersion compensations of the same. Since theaforementioned wavelength dispersion compensator 10 can supply theoptical signals, in which unevenness of the levels thereof are equalizedand the wavelength dispersions accumulated thereon are compensated, tothe optical transmission line 1, the quality of the signal transmissionof the optical transmission line 1 can be heightened.

[0076] Moreover, as shown in FIG. 5, a series connection of an opticalrepeater 15 and a gain equalizer 14 may be added to the wavelengthdispersion compensator 10 at the port 12-3 (the output terminal) of thecirculator 12 via an optical fiber or an optical waveguide. By providingthe optical repeater 15 for the wavelength dispersion compensator 10,some of the optical signals reflected from the fiber gratings of thereflection type 11, powers of which are lower than their rated valuesassigned by the level diagram, are amplified by the optical repeater 15to achieve their regular values.

[0077] Accordingly, in the wavelength dispersion compensator 10, thewavelength dispersions of the optical signals can be compensated, thepowers of the optical signals are compensated to achieve their regularvalues assigned by the level diagram, and their signal levels areequalized. Moreover, since the optical signals in which their powers arecompensated and their signal levels are equalized can be transmitted,the quality of the signal transmission of the optical transmission linescan be heightened.

[0078] Next, the operation of the wavelength dispersion compensator willbe explained referring to FIG. 3. In the WDM optical transmission, theoptical signals propagated through the optical transmission line 1mentioned later are supplied to the circulator 12 of the wavelengthdispersion compensator 10.

[0079] The optical signals inputted to the port 12-1 of the circulator12 are transmitted to the port 12-2, and supplied to the fiber gratingsof the reflection type 11. In the wavelength dispersion compensator 10shown in FIG. 3, the optical signal having a specified wavelength isreflected by one of the plural fiber gratings of the reflection type 11,again inputted to the port 12-2 (the input and output terminal), andoutputted from the port 12-3 (the output terminal) to the opticaltransmission line. Since the fiber gratings of the reflection type 11are situated at the different positions, there arises a difference inthe time spent in traveling to and from the fiber grating of thereflection type 11 between the optical signals. Accordingly, thewavelength dispersions of the optical signals can be satisfactorilycompensated by suitably determining the positions of the fiber gratingsof the reflection type 11.

[0080] As mentioned later, the optical signal λ₁ to λ₅ are transmittedfrom the optical transmitter (OS) 20 situated at the sending end, andthe wavelength dispersions of the optical signals are compensated by thewavelength dispersion compensator 10 in the lump. FIG. 6 shows adispersion map of the aforementioned system, and the wavelengthdispersions of the optical signals λ₁ to λ₅ are represented as functionsof the transmission distance. As shown in FIG. 6, zigzag linesrepresenting the wavelength dispersions of the respective opticalsignals coverage at a point A, which corresponds to the position of thewavelength dispersion compensator 10. It should be noted that thewavelength dispersions of the all the optical signals λ₁ to λ₅ becomezero at the point A.

[0081] When the dispersion map shown in FIG. 2 is compared with thatshown in FIG. 6, it can be clearly understood that the wavelengthdispersion compensator 10 according to the invention keeps thewavelength dispersions of the optical signals within a far narrowerrange than that of the conventional optical transmission systemthroughout a wide range of the wavelength. Accordingly, when thewavelength dispersion compensator 10 is applied to the opticaltransmission system, the optical signals can be transmitted under thelow wavelength dispersions, and the quality of the signal transmissioncan be heightened.

Optical Transmission Lines According to Referred Embodiments ofInvention

[0082] Next, an optical transmission line according to the inventionwill be explained referring to FIG. 7. FIG. 7 shows a structure of theoptical transmission line according to the preferred embodiment of theinvention schematically. As shown in this drawing, the opticaltransmission line 1 is composed of an optical transmitter (OS) 20, anoptical fiber for a signal transmission 30, optical repeaters 40, and anoptical receiver (OR) 50. Furthermore, a wavelength dispersioncompensator 10 is added thereto.

[0083] In the optical transmission line 1, dispersion compensationfibers 60 may be inserted into the optical fiber for the signaltransmission 30 in series at predetermined positions on the routethereof. In the optical transmission line 1, the wavelength dispersionsof the optical signals can be compensated surely by using the wavelengthdispersion compensator 10 in combine with the dispersion compensationfibers 60.

[0084] Moreover, the optical signals transmitted from the opticaltransmitter (OS) 20 can be transmitted to the optical receiver (OR) 50after the wavelength dispersions thereof have been compensated in thewavelength dispersion compensator 10 by providing the wavelengthdispersion compensator 10 for the optical transmission line 1.Accordingly, the quality of the signal transmission of the opticaltransmission line 1 can be heightened. The structure of the wavelengthdispersion compensator 10 can be selected from those shown in FIGS. 3 to5.

[0085] Moreover, as shown in FIG. 8, the optical transmission line 1 maybe provided with a demultiplexing arrayed waveguide grating (AWG,hereinafter) 70 a and a multiplexing AWG 70 b. The plural wavelengthdispersion compensators 10-1 to 10-n connected in parallel are insertedbetween the demultiplexing AWG 70 a and the multiplexing AWG 70 b.

[0086] Herein, each of the demultiplexing AWG 70 a and the multiplexingAWG 70 b is composed of the waveguides which are arranged thereon bydepositing SiO₂ on a Si substrate and arranged thereon to form an array.According to the aforementioned structure, since the diffraction angleof the optical signal in the slab waveguide varies as a function of thewavelength of the optical signal similarly to the diffraction gratings,the demultiplexed optical signals can be taken out from the outputwaveguides.

[0087] Each of the wavelength dispersion compensators 10-1 to 10 ncorresponds to one of the WDM optical signals diffracted by thedemultiplexing AWG 70 a. Accordingly, the wavelength dispersions of theWDM optical signals can be compensated throughout a wide range of thewavelength by the wavelength dispersion compensators 10-1 to 10-n as awhole. It should be noted that, although each of the dispersioncompensators 10-1 to 10-n is provided with a single fiber grating of thereflection type 11, optical distances between the fiber gratings of thereflection type 11 and the corresponding circulators 12 take differentvalues depending on the wavelengths of the reflected optical signals inthe plural wavelength dispersion compensators 10-1 to 10-n.

[0088] In the optical transmission line 1 provided with thedemultiplexing AWG 70 a and the multiplexing AWG 70 b, Co doped fibers80 may be inserted between the wavelength dispersion compensators 10-1to 10-n and the multiplexing AWG 70 b. The Co doped fibers 80 areinserted between the wavelength dispersion compensators 10-2 to 10-(n−1)outputting the optical signals having high signal levels and themultiplexing AWG 70 b.

[0089] If the optical transmission line 1 having the structure shown inFIG. 8 is adopted, since some of the optical signals outputted from thewavelength dispersion compensators 10-1 to 10-n, each having a highsignal level, are propagated through the Co doped fibers 80 and undergoinsertion losses, the levels of the optical signals to be multiplexed bythe multiplexing AWG 70 b can be equalized.

[0090] Moreover, in the optical transmission line 1 shown in FIG. 9, theoptical amplifiers 90-1 to 90-n may be inserted between the outputterminals of the wavelength dispersion compensators 10-1 to 10-n and themultiplexing AWG 70 b respectively.

[0091] If the optical transmission line is constructed in the structureshown in FIG. 9, the optical signals outputted from the wavelengthdispersion compensators 10-1 to 10-n can be so amplified by the opticalamplifiers 90-1 to 90-n that the peak levels of the optical signals areequalized. According to the aforementioned structure, deflections of thesignal levels in the respective channels can be corrected easily.

[0092] As mentioned in the above, according to the invention, in thefiber gratings of the reflection type provided for the wavelengthdispersion compensator, since the optical signals are reflected from thedifferent positions depending on the wavelengths of the incident opticalsignals, the wavelength dispersions of the respective optical signalscan be compensated because of the difference in the time spent intravelling to and from the fiber gating of the reflection type betweenthe optical signals. Accordingly, since the optical signals, thewavelength dispersions of which are satisfactorily compensated, aretransmitted to the optical receiver, the quality of the signaltransmission of the optical transmission line can be heightened.

[0093] Since the weight of the fiber gratings of the reflection type isless than that of the conventional dispersion compensation fiber and thelength thereof can be made short, the wavelength dispersion compensatorbecomes compact and lightweight, and the cost price thereof can bereduced. The wavelength dispersions of the optical signals can becompensated by a simple structure by providing the fiber gratings of thereflection type and the circulator for the wavelength dispersioncompensator.

[0094] The fiber grating of the reflection type can be fabricated forthe optical signal having a wavelength selected at will and a wavelengthdispersion to be compensated having any desired value. Moreover, thewavelength dispersion extending over a wide range can be compensated byadopting the chirped fiber grating, in which the wavelength of thereflected optical signal varies continuously in the longitudinaldirection.

[0095] Unevenness of the levels of the optical signals reflected fromthe fiber gratings of the reflection type can be equalized by insertingthe gain equalizer between the output terminal of the circulator and theoptical transmission line. Moreover, some of the optical signalsreflected from the fiber gratings of the reflection type, the powerlevels of which are lower than their rated values assigned by the leveldiagram, can be compensated by the optical repeater inserted between thecirculator and the gain equalizer.

[0096] Since the wavelength dispersion compensator having the fibergratings of the reflection type is provided for the optical transmissionline, the insertion losses caused by increases of the dispersioncompensations are lower than those of the conventional dispersioncompensation fiber, and thereby the loss compensation amplifier becomesunnecessary, hence the cost prices of the optical transmission line andthe wavelength dispersion compensator can be cut down.

[0097] Since the optical transmission system is composed of the pluralwavelength dispersion compensators corresponding to the respectiveoptical signals which are propagated through the optical transmissionline and undergo the wavelength dispersions, the demultiplexing AWGwhich demultiplexes the WDM optical signals into the plural opticalsignals and supply them to the plural wavelength dispersion compensatorsrespectively, and the multiplexing AWG which multiplexes the respectiveoptical signals outputted from the plural wavelength dispersioncompensators, the wavelength dispersions of the optical signals arecompensated throughout a wide range of the wavelength by the pluralwavelength dispersion compensators, each of which is suitably designedto serve as a compensator for an optical signal inputted thereto.

[0098] Moreover, since the Co doped fibers are inserted between thewavelength dispersion compensators and the multiplexing AWG, some of theoptical signals outputted from the wavelength dispersion compensators,each having a high signal level, are propagated through the Co dopedfibers, and undergo level losses, hence the signal levels of therespective optical signals can be equalized.

[0099] Moreover, since the optical amplifiers are inserted between thewavelength dispersion compensators and the multiplexing AWG, the opticalsignals, the wavelength dispersions accumulated on which have beencompensated by the wavelength dispersion compensators, are so amplifiedby the optical amplifiers that the peak levels thereof are equalized,and transmitted to the multiplexing AWG, hence the deflections of thelevels of the optical signals can be corrected easily.

[0100] Although the invention has been described with respect tospecific embodiment for complete and clear disclosure, the appendedclaims are not to be thus limited but are to be construed as embodyingall modification and alternative constructions that may be occurred toone skilled in the art which fairly fall within the basic teachingherein set forth.

1. A wavelength dispersion compensator for compensating for wavelengthdispersions of wavelength division multiplexed (WDM) optical signalswhich have been propagated through an input optical transmission line,comprising: plural fiber gratings of a reflection type which reflectsaid WDM optical signals at different positions depending on wavelengthsof said reflected optical signals, and a gain equalizer having an inputterminal for receiving said reflected optical signals and an outputterminal connected to one of an output optical transmission line and anoutput optical waveguide.
 2. The wavelength dispersion compensator asdefined in claim 1, further comprising: a circulator comprising an inputterminal, an input and output terminal, an an output terminal, saidinput terminal being connected with said input optical transmissionline, said input and output terminal being in communication with saidplural fiber gratings of a reflection type, and said output terminalbeing connected with one of said output optical transmission line andsaid output optical waveguide.
 3. The wavelength dispersion compensatoras defined in claim 2, wherein said plural fiber gratings of areflection type comprise a chirped fiber grating of a reflection type.4. The wavelength dispersion compensator as defined in claim 2, whereinan input terminal of said gain equalizer is connected with said outputterminal of said circulator, and an output terminal of said gainequalizer is connected with said output optical transmission line orsaid output optical waveguide. 5-11. (Canceled)
 12. The wavelengthdispersion compensator as defined in claim 1, wherein said plural fibergratings are connected in series by one of an optical fiber and anoptical waveguide.
 13. The wavelength dispersion compensator as definedin claim 12, wherein said series comprises a cascading series.
 14. Thewavelength dispersion compensator as defined in claim 1, wherein saidplural fiber gratings comprise a chirped fiber grating having abandwidth of several to several tens of nanometers.
 15. The wavelengthdispersion compensator as defined in claim 2, wherein said inputterminal, said input and output terminal, and said output terminal arearranged symmetrically around said circulator, and a direction ofpropagation of said optical signals is in a direction away from saidoutput terminal and toward said plural fiber gratings.
 16. Thewavelength dispersion compensator as defined in claim 2, wherein saidcirculator comprises four or more terminals.
 17. The wavelengthdispersion compensator as defined in claim 2, wherein said opticalsignals are input directly to one of said plural fiber gratings from oneof said circulator and another one of said plural fiber gratings. 18.The wavelength dispersion compensator as defined in claim 1, whereinrefractive indices of said plural fiber gratings vary periodically as afunction of longitudinal distance, and a period of variation is lessthan 1 μm.
 19. The wavelength dispersion compensator as defined in claim1, wherein an optical signal reflected by a fiber grating in said pluralfiber gratings has a wavelength, λ_(B), given by Equation (1), λ_(B)=2n_(eff) A(μm)  (1) wherein n_(eff) comprises an effective refractiveindex of an optical fiber in said fiber grating, and A comprises acoefficient determined by a variation of the refractive index of saidoptical fiber.
 20. A method of compensating for wavelength dispersionsof wavelength division multiplexed (WDM) optical signals, said methodcomprising: reflecting said WDM optical signals at different positionsdepending on wavelengths of said reflected optical signals, using pluralfiber gratings, wherein a number of said plural fiber gratings which isat least equivalent to a number of said optical signals are connected inseries.